In the art of planographic printing, it is the common practice to use a metal plate to which is imparted on one surface thereof water retentivity or a hydrophilic property by a graining or anodizing method and then coated an olephilic photosensitive composition. However, the prior metal plate for printing involves a number of disadvantages. One of the disadvantages is that the manufacturing process is complicate and thus the plate is expensive in production. Another disadvantage is that the making of a printing plate essentially requires relatively difficult, delicate steps such as a lithographic film-making step, a developing step, etc., presenting difficulties in planographic printing.
In order to overcome the above disadvantages, there have been proposed several plates for planographic printing or methods for easily making such plates which are usable in plate making directly from an electrical signal converted from an intended image. Such a plate is disclosed, for example, in Japanese laid-open Patent Application No. 124708/1975, in which an oleophilic substrate principally made of an oleophilic resin is subjected to corona discharge to form a hydrophilic layer on one surface of the substrate. For example, the printing plate making can be done directly by selectively destroying or removing areas of the hydrophilic layer corresponding to an intended image by a mechanical, thermal or electrical method. The plate for planographic printing of the type just mentioned has various advantages such as easy printing plate making, good reproducibility of image and the like. However, the printing plate suffers from objectionable disadvantages mainly due to lack of hydrophilic property of the hydrophilic layer formed on one surface of the substrate by corona discharge: the printing plate has a low durability of, for example, less than several thousands prints and there is a relatively severe limitation in the kind of ink employable.